1. Field of the Invention
The present invention relates to plug valves and, more particularly, to tapered plug valves.
2. Description of the Prior Art
Tapered plug valves are found in a wide range of uses in various flow systems including systems for conveying or transporting slurries. More particularly, tapered plug valves have been found particularly adapted to systems in which a solid is conveyed in a liquid as, for example, coal, copper, limestone, iron ore or the like suspended in a liquid medium, typically water. Further, coal may be conveyed in a piping system suspended in oil. It is important in this type of system and other flow systems that the valve be particularly rugged and reliable and, in the case of a tapered plug valve, it is important that it not be subject to valve lockup.
Experimentation has been directed to reducing and/or eliminating the lockup phenomenon as applied to tapered plug valves. The basic thrust of this experimentation is directed to the elimination of the interference lockup or pinching believed to be critical when a tapered plug valve is in the open or partially open position. It has been found and is generally believed that a pressure pulse in the line in which the valve is connected and/or the weight of the plug when unfavorably orientated will first cause sticking; then if the line pressure is reduced, an interface pressure is created which will give rise to a high frictional resistance between the casing and the plug thus rendering the valve extremely difficult to close. The need has become apparent, therefore, for a tapered plug valve in which hydraulic and/or gravitational forces exerted on the plug with a finite component toward the cone or taper apex are eliminated.